Compile mixed models

[richfitz]

Follows from #102, and requires mrc-ide/mode#24 and mrc-ide/odin#261 to be merged first

Support for compilation of simple mixed models, probably enough for us to implement the malaria model completely

[codecov]

Codecov Report

Merging #103 (7e593f4) into master (54abe06) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 7e593f4 differs from pull request most recent head a38bf97. Consider uploading reports for the commit a38bf97 to get more accurate results

@@            Coverage Diff            @@
##            master      #103   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines         1025      1039   +14     
=========================================
+ Hits          1025      1039   +14     

Impacted Files	Coverage Δ
R/generate_dust.R	100.00% <100.00%> (ø)
R/generate_dust_equation.R	100.00% <100.00%> (ø)

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